Iterative multi-stage detection technique for a diversity receiver having multiple antenna elements

ABSTRACT

An iterative multistage detection system and method for orthogonally multiplexing K channels onto a signal processing chain using N orthogonal sequences of length N. The K channels include a first set of N channels and a second set of M channels (the M channels being separate and distinct from the N channels), where K=N+M. In a first iteration, interference from the first set of N channels imparted on the second set of M channels is removed from the multiplexed signal, thereby enabling the symbol values associated with the second set of M channels to be reliably estimated. In a second iteration, interference from the second set of M channels imparted on the first set of N channels is removed from the first set of N channels, thereby enabling the symbol values associated with the first set of N channels to be reliably estimated.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Application No. 60/407,524 entitled ITERATIVE MULTI-STAGEDETECTION TECHNIQUE FOR DIVERSITY RECEIVER HAVING MULTIPLE ANTENNAELEMENTS, filed Aug. 28, 2002, which is incorporated herein by referencein its entirety

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an antenna diversity receiverfor radio communication systems, and more particularly to amulti-channel detection process implemented in a receiver receivingsignals over multiple channels.

[0004] 2. Background Information

[0005] It has recently been proposed that both the performance andcapacity of existing wireless systems could be improved through the useof so-called “smart” antenna techniques. In particular, it has beensuggested that such techniques, coupled with space-time signalprocessing, could be utilized both to combat the deleterious effects ofmultipath fading of a desired incoming signal and to suppressinterfering signals. In this way both performance and capacity ofdigital wireless systems in existence or being deployed (e.g.,CDMA-based systems, TDMA-based systems, WLAN systems, and OFDM-basedsystems such as IEEE 802.11a/g) may be improved.

[0006] It is anticipated that smart antenna techniques will beincreasingly utilized both in connection with deployment of base stationinfrastructure and mobile subscriber units (e.g, handsets) in cellularsystems in order to address the increasing demands being placed uponsuch systems. These demands are arising in part from the shift underwayfrom current voice-based services to next-generation wireless multimediaservices and the accompanying blurring of distinctions among voice,video and data modes of transmission. Subscriber units utilized in suchnext-generation systems will likely be required to demonstrate highervoice quality relative to existing cellular mobile radio standards aswell as to provide high-speed data services (e.g., as high as 10Mbits/s). Achieving high speed and high quality of service, however, iscomplicated because it is desireable for mobile subscriber units to besmall and lightweight, and to be capable of reliably operating in avariety of environments (e.g., cellular/microcellular/picocellular,urban/suburban/rural and indoor/outdoor). Moreover, in addition tooffering higher-quality communication and coverage, next-generationsystems are desired to more efficiently use available bandwidth and tobe priced affordably to ensure widespread market adoption.

[0007] In many wireless systems, three principal factors tend to accountfor the bulk of performance and capacity degradation: multipath fading,delay spread between received multipath signal components, andco-channel interference (CCI). As is known, multipath fading is causedby the multiple paths which may be traversed by a transmitted signal enroute to a receive antenna. The signals from these paths add togetherwith different phases, resulting in a received signal amplitude andphase that vary with antenna location, direction and polarization, aswell as with time (as a result of movement through the environment).Increasing the quality or reducing the effective error rate in order toobviate the effects of multipath fading has proven to be extremelydifficult. Although it would be theoretically possible to reduce theeffects of multipath fading through use of higher transmit power oradditional bandwidth, these approaches are often inconsistent with therequirements of next-generation systems.

[0008] As mentioned above, the “delay spread” or difference inpropagation delays among the multiple components of received multipathsignals has also tended to constitute a principal impediment to improvedcapacity and performance in wireless communication systems. It has beenreported that when the delay spread exceeds approximately ten percent(10%) of the symbol duration, the resulting significant intersymbolinterference (ISI) generally limits the maximum data rate. This type ofdifficulty has tended to arise most frequently in narrowband systemssuch as the Global System for Mobile Communication (GSM).

[0009] The existence of CCI also adversely affects the performance andcapacity of cellular systems. Existing cellular systems operate bydividing the available frequency channels into channel sets, using onechannel set per cell, with frequency reuse. Most time division multipleaccess (TDMA) systems use a frequency reuse factor of 7, while most codedivision multiple (CDMA) systems use a frequency reuse factor of 1. Thisfrequency reuse results in CCI, which increases as the number of channelsets decreases (i.e., as the capacity of each cell increases). In TDMAsystems, the CCI is predominantly from one or two other users, while inCDMA systems there may exist many strong interferers both within thecell and from adjacent cells. For a given level of CCI, capacity can beincreased by shrinking the cell size, but at the cost of additional basestations.

[0010] The impairments to the performance of cellular systems of thetype described above may be at least partially ameliorated by usingmulti-element antenna systems designed to introduce a diversity gaininto the signal reception process. There exist at least three primarymethods of effecting such a diversity gain through decorrelation of thesignals received at each antenna element: spatial diversity,polarization diversity and angle diversity. In order to realize spatialdiversity, the antenna elements are sufficiently separated to enable lowfading correlation. The required separation depends on the angularspread, which is the angle over which the signal arrives at the receiveantennas.

[0011] In the case of mobile subscriber units (e.g, handsets) surroundedby other scattering objects, an antenna spacing of only one quarterwavelength is often sufficient to achieve low fading correlation. Thispermits multiple spatial diversity antennas to be incorporated within ahandset, particularly at higher frequencies (owing to the reduction inantenna size as a function of increasing frequency). Furthermore, dualpolarization antennas can be placed close together, with low fadingcorrelation, as can antennas with different patterns (for angle ordirection diversity).

[0012] Although increasing the number of receive antennas enhancesvarious aspects of the performance of multi-antenna systems, thenecessity of providing a separate RF chain for each transmit and receiveantenna increases costs. Each RF chain is generally comprised of a lownoise amplifier, filter, downconverter, and analog to digital toconverter (A/D), with the latter three devices typically beingresponsible for most of the cost of the RF chain. In certain existingsingle-antenna wireless receivers, the single required RF chain mayaccount for in excess of 30% of the receiver's total cost. It is thusapparent that as the number of receive antennas increases, overallsystem cost and power consumption may dramatically increase. It wouldtherefore be desirable to provide a technique that effectively providesadditional receive antennas without proportionately increasing systemcosts and power consumption.

SUMMARY OF THE INVENTION

[0013] In one embodiment, the invention can be characterized as amethod, and means for accomplishing the method, for receiving a signal,the method including receiving K replicas of the signal, each of the Kreplicas being received by one of a corresponding K antennas so as tothereby generate K received signal replicas; processing each of the Kreceived signal replicas using one of N orthogonal sequences, therebygenerating K processed signal replicas, wherein N is less than K;orthogonally multiplexing the K processed received signal replicas intoa multiplexed signal provided to a signal processing chain;downconverting, within the signal processing chain, the multiplexedsignal into a baseband multiplexed signal; and transforming the basebandmultiplexed signal into K separate signals wherein each of the Kseparate signals corresponds to one of the K replicas of the signal.

[0014] In another embodiment, the invention may be characterized asapparatus for receiving a signal comprising: K antenna elements, whereinthe K antenna elements are arranged to receive one of a corresponding Kreplicas of the signal and thereby generate K received signal replicas;a signal processing chain; a first multiplexer configured to receive Nof the K received signal replicas and generate a first set of N channelsignals, wherein each of the N channel signals is spread according to acorresponding one of N orthogonal sequences and corresponds to one ofthe N received signal replicas; a second multiplexer configured toreceive M of the K received signal replicas and generate a second set ofM channel signals, wherein each of the M channel signals is spreadaccording to one of the N orthogonal sequences and corresponds to one ofthe M received signal replicas; a summing portion coupled between thesignal processing chain and the first and second multiplexers, whereinthe summing portion is configured to combine the first set of N channelsignals and the second set of M channel signals into a multiplexedsignal and provide the multiplexed signal to the signal processingchain; a downconversion module configured to downconvert, within thesignal processing chain, the multiplexed signal to a basebandmultiplexed signal; and a signal recovery module coupled to the signalprocessing chain, wherein the signal recovery module is configured toreceive the baseband multiplexed signal and provide K separate signalsfrom the baseband multiplexed signal, wherein each of the K separatesignals corresponds to one of the K replicas of the signal.

[0015] In a further embodiment, the invention may be characterized as amethod for multiplexing K channels on to a receiver chain, the Kchannels including N channels corresponding to N antenna elements and Mchannels corresponding to M antenna elements, the method comprising:spreading each of the N channels according to a corresponding one of Northogonal sequences so as to form N spread channels; overlaying a firstscrambling sequence on to the N spread channels so as to form a firstset of N channels; spreading each of the M channels according to one ofthe N orthogonal sequences so as to form M spread channels; overlaying asecond scrambling sequence on to the M spread channels so as to form asecond set of M channels; combining the first set of N channels and thesecond set of M channels so as to form K multiplexed channels; andproviding the K multiplexed channels to the receiver chain.

[0016] In yet another embodiment the invention may be characterized as amethod for separating K symbol streams, each of the K symbol streamsbeing conveyed by K respective orthogonally spread channels in areceiver chain, the K channels including a first set of N channels and asecond set of M channels, each of the N channels being spread accordingto a corresponding one of N orthogonal sequences and each of the Mchannels being spread according to one of the N orthogonal sequences,the method comprising: despreading the first set of N channels so as togenerate N separate channels; detecting, from the N separate channels, aset of N symbols wherein each of the N symbols is conveyed by acorresponding one of the N channels; generating a first interferencesignal due to the first set of N channels based upon the set of Nsymbols; subtracting the interference signal from the second set of Mchannels; despreading the second set of M channels so as to generate Mseparate channels; detecting, from the M separate channels, a set of Msymbols wherein each of the M symbols is conveyed by a corresponding oneof the M channels; and providing K separate symbols wherein the Kseparate symbols include the set of N symbols and the set of M symbols.

[0017] In yet a further embodiment, the invention may be characterizedas a method for receiving a signal with an antenna array comprising:receiving K replicas of the signal, each of the K replicas beingreceived by one of a corresponding K antenna elements of the antennaarray, wherein the K replicas include N replicas and M other replicas ofthe received signal; multiplexing the N replicas and the M replicas ofthe signal into a multiplexed signal provided to a single processingchain; removing interference due to the N signals from the multiplexedsignal; demultiplexing, after the interference due to the N signals isremoved, the M signals from the multiplexed signal, thereby generating Mdetected signals; removing interference due to the M signals from themultiplexed signal; demultiplexing, after the interference due to the Msignals is removed, the N signals from the multiplexed signal, therebygenerating N detected signals.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In the accompanying drawings:

[0019]FIG. 1 is a block diagram of a conventional diversity receiver inwhich the signals received by multiple antenna elements are weighted andcombined in order to generate an output signal;

[0020]FIG. 2 is a block diagram of a conventional spatial-temporal (ST)filtering arrangement;

[0021]FIG. 3 is a representation of a multiple-input/multiple-outputantenna arrangement within a wireless communication system;

[0022]FIG. 4 is a block diagram of an antenna processing systemconfigured to reduce the number of separate signal processing chainsassociated with an antenna array;

[0023]FIG. 5 is a high-level block diagram of a multi-antenna receiversystem implemented in accordance with one embodiment of the presentinvention;

[0024]FIG. 6 is a flow chart illustrating steps carried out by the amulti-antenna receiver system of FIG. 5 to receive a signal withmultiple antennas according to one embodiment;

[0025]FIG. 7 is a block diagram of a multi-antenna receiver systemconfigured to implement iterative multi-stage detection in accordancewith one embodiment of the antenna system of FIG. 5;

[0026]FIG. 8 is a flowchart depicting steps carried out by themulti-antenna receiver system of FIG. 7 when carrying out the iterativemultistage detection process according to one embodiment of the presentinvention;

[0027]FIG. 9 is a graph depicting simulated results of the iterativemultistage detection process carried out by the multi-antenna receiversystem of FIG. 7 with a spreading factor of (N) selected to be 16 and anumber of channels selected to be N+1;

[0028]FIG. 10 is a graph depicting simulated results of the iterativemultistage detection process carried out by the multi-antenna receiversystem of FIG. 7 with a spreading factor of (N) selected to be 16 and anumber of channels selected to be N+2;

[0029]FIG. 1I is a graph depicting simulated results of the iterativemultistage detection process carried out by the multi-antenna receiversystem of FIG. 7 with a spreading factor of (N) selected to be 16 and anumber of channels selected to be N+3;

[0030]FIG. 12 is a graph depicting simulated results of the iterativemultistage detection process carried out by the multi-antenna receiversystem of FIG. 7 with a spreading factor of (N) selected to be 16 and anumber of channels selected to be N+7; and

[0031]FIG. 13 is a graph depicting simulated results of the iterativemultistage detection process carried out by the multi-antenna receiversystem of FIG. 7 with a spreading factor of (N) selected to be 7 and anumber of channels selected to be N+1.

DETAILED DESCRIPTION OF THE INVENTION

[0032] In the following description, various aspects of the presentinvention will be described. However, it will be apparent to thoseskilled in the art that the present invention may be practiced with onlysome or all aspects of the present invention. For purposes ofexplanation, specific numbers, materials and configurations are setforth in order to provide a thorough understanding of the presentinvention. However, it will also be apparent to one skilled in the artthat the present invention may be practiced without the specificdetails. In other instances, well known features are omitted orsimplified in order not to obscure the present invention.

[0033] Various operations will be described as multiple discrete stepsperformed in turn in a manner that is most helpful in understanding thepresent invention, however, the order of description should not beconstrued as to imply that these operations are necessarily orderdependent, in particular, the order the steps are presented.Furthermore, the phrase “in one embodiment” will be used repeatedly,however the phrase does not necessarily refer to the same embodiment,although it may.

[0034] The present invention according to several embodiments allows Ksignal channels associated with K respective antenna elements to beorthogonally multiplexed onto a signal processing chain of a receiverusing less than K orthogonal sequences. As a consequence, a receiverusing a single receive chain characterized by a spreading factor of N,which would otherwise be limited to N antenna elements, may incorporatemore than N antenna elements; thus increasing the capacity of thereceiver.

[0035] The present invention is applicable to mobile devices and alsoinfrastructure elements (e.g., base stations and access points). Inaddition, the present invention is applicable to nearly all knownwireless standards and modulation schemes (e.g., GSM, CDMA2000, WCDMA,WLAN, fixed wireless standards, OFDM and CDMA). As will be describedbelow, various advantages offered by the present invention derive fromthe multiplexing of the signals received from a number of antennaelements onto a common receive chain processing path in order to reduceoverall power consumption and cost.

[0036] In order to facilitate appreciation of the principles of theinvention, a brief overview of various conventional multi-elementantenna systems designed to mitigate delay spread, interference andfading effects is provided with reference to FIGS. 1-4.

[0037] Referring first to FIG. 1, shown is a block diagram of aconventional diversity receiver 100 in which the signals received bymultiple antenna elements are weighted and combined in order to generatean output signal. Shown in the conventional diversity receiver 100 are acollection of M antenna elements 102, and coupled with each respectiveantenna element are parallel receive chains 104, 106, 108 that includerespective weighting portions 110, 112, 114. The receive chains 104,106, 108 all couple with a combiner 116 disposed to produce a combinedsingle 118.

[0038] An array of M antenna elements generally provides an increasedantenna gain of “M.” Such an array also provides a diversity gainagainst multipath fading dependent upon the correlation of the fadingamong the antenna elements. In this context the antenna gain is definedas the reduction in required receive signal power for a given averageoutput signal-to-noise ratio (SNR), while the diversity gain is definedas the reduction in the required average output SNR for a given biterror rate (BER) with fading.

[0039] For interference mitigation, each of the M antenna elements 102are weighted at the respective weighting portions 110, 112, 114 andcombined in the combiner 116 to maximizesignal-to-interference-plus-noise ratio (SINR). This weighting processis usually implemented in a manner that minimizes mean squared error,and utilizes the correlation of the interference to reduce theinterference power.

[0040] Turning now to FIG. 2, a block diagram is shown of a conventionalspatial-temporal (ST) filtering arrangement 200. Shown are a firstantenna 202 and a second antenna 204 respectively coupled to a firstlinear equalizer 206 and a second linear equalizer 208. Outputs of eachof the first and second linear equalizers 206, 208 are coupled to acombiner 210, and an output of the combiner 201 is coupled to anMLSE/DFE portion 212.

[0041] The filtering arrangement of FIG. 2 is designed to eliminatedelay spread using joint space-time processing. In general, since theCCI is unknown at the receiver, optimum space-time (ST) equalizers,either in the sense of a minimum mean square error (MMSE) or maximumsignal-to-interference-plus-noise ratio (SINR), typically include awhitening filter. For example, linear equalizers (LE) 206, 208 thatwhiten the CCI both spatially and temporally, and the filteringarrangement of FIG. 2 are typical of such systems. As shown in FIG. 2,the linear equalizers (LE) 206, 208 are followed by a non-linear filterthat is represented by the MLSE/DFE portion 212, which is implementedusing either a decision feedback equalizer (DFE) or maximum-likelihoodsequence estimator (MLSE).

[0042] As is known to one of ordinary skill in the art, the turboprinciple can also be used to replace the non-linear filters withsuperior performance, but higher computational complexity. Using STprocessing (STP) techniques, SNR gains of up to 7 dB and SINR gains ofup to 21 dB have been reported with a modest number of antenna elements.

[0043] Referring next to FIG. 3, shown is a generic representation of amultiple-input/multiple-output antenna arrangement within a wirelesscommunication system 300. Shown are a transmitter (TX) 302 coupled tomultiple transmit antennas 304, which are shown transmitting a signalvia time varying obstructions 306 to multiple receive antennas 308coupled to a receiver (RX) 310.

[0044] In addition to multiple-input/multiple-output antenna (MIMO)arrangements, other antenna arrangements may be categorized, based uponthe number of “inputs” and “outputs” to the channel linking atransmitter and receiver, as follows: Single-input/single-output (SISO)systems, which include transceivers (e.g., mobile units and a basestation) with a single antenna for uplink and down link communications.

[0045] Multi-input/single-output (MISO) systems, which include one ormore receivers, which downlink via multiple antenna inputs, and one ormore transmitters, which uplink via a single antenna output.Single-input/multi-output (SIMO) systems, which include one or morereceivers, which downlink via a single antenna input, and one or moretransmitters, which uplink via multiple antenna outputs.

[0046] One aspect of the attractiveness of multi-element antennaarrangements, particularly MIMOs, resides in the significant systemcapacity enhancements that can be achieved using these configurations.Assuming perfect estimates of the applicable channel at both thetransmitter and receiver are available, in a MIMO system with M receiveantennas the received signal decomposes to M independent channels. Thisresults in an M-fold capacity increase relative to SISO systems. For afixed overall transmitted power, the capacity offered by MIMOs scalewith increasing SNR for a large, but practical, number of M of antennaelements.

[0047] In the particular case of fading multipath channels, it has beenfound that the use of MIMO arrangements permits capacity to be scaled bynearly M additional bits/cycle for each 3 -dB increase in SNR. This MIMOscaling attribute is in contrast to a baseline configuration,characterized by M=1, which by Shannon's classical formula scales as onemore bit/cycle for every 3-dB of SNR increase. It is noted that thisincrease in capacity that MIMO systems afford is achieved without anyadditional bandwidth relative to the single element baselineconfiguration.

[0048] However, widespread deployment of multi-element antennaarrangements in wireless communication systems (particularly withinwireless handsets) has been hindered by the resultant increase incomplexity and associated increased power consumption, cost and size.These parameter increases result, at least in part, from a requirementin many proposed architectures that a separate receiver chain beprovided for each antenna element.

[0049] One technique which has been developed to utilize multipleantenna elements with a reduced number of signal processing chainsincludes multiplexing signals from multiple antennas on to a singleprocessing chain as disclosed in a related copending U.S. applicationSer. No. 10/606,371, entitled REDUCED-COMPLEXITY ANTENNA SYSTEM USINGMULTIPLEXED RECEIVE CHAIN PROCESSING, filed Jun. 27, 2003, which isassigned to the assignee of the present application and is incorporatedherein by reference in its entirety.

[0050] Referring next to FIG. 4, shown is an antenna processing system400 configured to reduce the number of separate signal processing chainsassociated with an antenna array in accordance with the above-identifiedU.S. application Ser. No. 10/606,371. As shown, the antenna processingsystem 400 includes N antennas 402, 404, 406, 408 coupled to amultiplexer 410, which is coupled to a single signal processing chain416. The multiplexer 410 is configured to orthogonally multiplex Nchannels (corresponding to the N antennas 402 onto the signal processingchain 416, and is characterized by a spreading factor of N: that is, themultiplexer 410 utilizes N orthogonal sequences of length N.

[0051] In operation, each of the N antennas 402 receives an incident RFsignal at spatially distinct locations and provides a replica of theincident RF signal to the multiplexer 410. As a consequence, themultiplexer 410 receives N replicas of the incident RF signal. Themultiplexer 410 then orthogonally multiplexes the N replicas of theincident RF signal on to the single processing chain 416 to form amultiplexed signal comprising N multiplexed channels. Because each ofthe N channels is assigned a different orthogonal code duringmultiplexing, a manageable level of interference exists between the Nmultiplexed channels within the signal processing chain 416.

[0052] Once provided to the signal processing chain 416, the multiplexedsignal is then frequency downconverted, filtered and converted fromanalog form into a digital multiplexed signal. The digital multiplexedsignal is then demultiplexed by a demultiplexor 436 into N separatesignals that correspond to the N replicas of the signal received at theN antennas. The N separate signals are then subjected to conventionalspatial processing.

[0053] Although the antenna processing system 400 provides substantialcost and power savings over systems employing a separate signalprocessing chain for each antenna, in some applications it would bedesirable if the antenna processing system 400 could support more than Nchannels, i.e., more than N antennas. Because each of the N orthogonalsequences is already used by one of the N antennas, however, anadditional channel multiplexed onto the signal processing chain 416would not be orthogonal to at least one of the N multiplexed channels.As a consequence, the additional channel would both impart deleteriousinterference on one or more of the N multiplexed channels and receivesubstantial interference from at least one of the N multiplexedchannels.

Overview

[0054] As is described in further detail below, the iterative multistagedetection technique of the present invention may be utilized to providea cost effective means to increase the capacity of wireless systemsdeploying multi-element antenna arrangements. In one aspect of theinvention, an antenna system is configured to orthogonally multiplex Kchannels onto a single signal processing chain using N orthogonalsequences of length N. The K channels include a first set of N channelsand a second set of M channels (the M channels being separate anddistinct from the N channels), where K=N+M and in an exemplaryembodiment M<N. Therefore, a multiplexed signal is created on the signalprocessing chain, which includes a first set of N multiplexed channelsand a second set of M multiplexed channels.

[0055] In accordance with one aspect of the invention, an iterativeprocess is used to receive the multiplexed signal. In a first iteration,interference from the first set of N channels imparted on the second setof M channels is removed from the multiplexed signal, thereby enablingthe symbol values associated with the second set of M channels to bereliably estimated. In a second iteration, interference from the secondset of M channels imparted on the first set of N channels is removedfrom the first set of N channels, thereby enabling the symbol valuesassociated with the first set of N channels to be reliably estimated. Inthis way, K channels may be multiplexed on to a single receiver chainwith less than K orthogonal sequences, and then reliably estimated afterprocessing (e.g., after down conversion and digitization) by thereceiver chain.

[0056] Referring next to FIG. 5, shown is a high-level block diagram ofa receiver 500 incorporating an antenna system in accordance with anexemplary embodiment of the present invention. While referring to FIG. 5simultaneous reference will be made to FIG. 6, which is a flow chartillustrating steps carried out by the antenna system 500 to receive asignal with multiple antennas according to the present embodiment. Asshown, the antenna system 500 includes an N channel multiplexer 502, andan M channel multiplexer 507. The N channel multiplexer 502 isconfigured to receive N replicas of a signal with a set of N respectiveantennas 505, and the M channel multiplexer 507 is configured to receiveM replicas of the signal with a set of M respective antennas 508.Collectively the N and M channel multiplexers 502, 507 receive K signalreplicas (i.e., K=M+N) (Step 600).

[0057] In operation, the N channel multiplexer 502 and the M channelmultiplexer 507 collectively multiplex, in cooperation with thesummation module 530, the K received signal replicas on to the signalprocessing chain 510 (Step 602). In an exemplary embodiment, the Nchannel multiplexer 502 assigns each of the N replicas of the signal acorresponding one of N orthogonal time sequences to form a firstcomposite signal. The N channel multiplexer 502 then overlays a commonfirst PN scrambling sequence on to the first composite signal so as toform a first set of N scrambled signals 512 (also referred to herein asa “first set of N channels” or “set #1 channels”).

[0058] Similarly, the M channel multiplexer 507 assigns each of M of theN orthogonal sequences to a corresponding one of the M replicas of thesignal to form a second composite signal. In other words, the M channelmultiplexer 507 reuses a subset of the N orthogonal sequences to formthe second composite signal. The second multiplexer 507 then overlays asecond PN scrambling sequence on to the second composite signal so as toform a second set of M scrambled signals 514 (also referred to herein asa “second set of M channels” or “set #2 channels”). The summation module530 then combines the first set of N channels 512 and second set of Mchannels 514 so as to form a multiplexed signal 516, which is providedto the signal processing chain 510. Within the signal processing chain510 the multiplexed signal 516 is downconverted by a downconversionmodule 540 (e.g., a mixer to convert from RF to baseband frequency),filtered by a filter 542 and digitized by an analog to digital converter544.

[0059] Assuming time synchronization is established throughout theantenna system 500, there exists substantially no mutual interference inthe processing chain 510 among the first set of N channels. That is, thefirst set of N channels only experience interference as a consequence ofthe second set of M channels. The interference power (i.e., in-phase andquadrature phase energy) associated with each channel of the second setof M channels (assuming that useful signal power is normalized by 1) is1/N. It follows that the total interference power experienced by thefirst set of N channels is MIN. As long as M remains relatively smallcompared to N it is possible to make at least preliminary decisions asto the values of the symbols transmitted via the first set of Nchannels. However, since each channel of the second set of M channelsexperiences an interference power of N (1/N) or 1 as a consequence ofthe first set of N channels, the symbol values associated with thesecond set of M channels may not be directly estimated with anyreasonable degree of certainty through straightforward application ofconventional techniques.

[0060] As shown in FIG. 5, after the multiplexed signal 516 isdownconverted, filtered and digitized, the resultant basebandmultiplexed signal 546 is provided to a signal recovery module 550. Ingeneral, the signal recovery module 550 receives the basebandmultiplexed signal 546 and recovers K separate signals, which correspondto the K received signal replicas received by the K antennas.

[0061] Initially, the signal recovery module 550 receives the basebandmultiplexed signal 546, and removes interference imparted by the firstset of N channels on the second set of M channels from the multiplexedsignal so as to generate a preliminary estimate of the symbol streamscarried by the second set of M channels (Step 604). In an exemplaryembodiment, the signal recovery module 550 determines the interferenceimparted by the first set of N channels upon the second set of Mchannels by demultiplexing the first set of N channels from the basebandmultiplexed signal 546, establishing preliminary values of the symbolsreceived through the first set of N channels and then synthesizing anaggregate interference signal associated with the first set of Nchannels based upon these preliminary symbol values. The aggregateinterference signal also provides an estimate of the symbol streamsconveyed via the first set of N channels.

[0062] After interference from the first set of N channels is removedfrom the baseband multiplexed signal 546, the signal recovery module 550demultiplexes M separate signals (corresponding to the M replicas of thesignal) from the preliminary estimate of the second set of M channels(Step 606). Because interference from the first set of N channels isfirst removed from the baseband multiplexed signal 546 to form thepreliminary estimate of the second set of M channels, the signalrecovery module 550 may reliably estimate the symbol values associatedwith the M separate signals.

[0063] During a second signal recovery iteration, interference from thesecond set of M channels is then removed from the estimates of thesymbol streams corresponding to the first set of N channels (producedduring Step 604) in order to provide a revised estimate of these symbolstreams (Step 608). Since the preliminary symbol values of the first setof N channels are initially made in the presence of the interferencefrom the second set of M channels, this step removes the interferenceoriginating from the second set of M channels so the symbol values ofthe first set of N channels may be more reliably estimated.

[0064] The signal recovery module 550 then demultiplexes the revisedestimate of the first set of N channels into N separate signals(corresponding to the N replicas of the incident RF signal) from thebaseband multiplexed signal 546 (Step 610).

[0065] The signal recovery module 550 then provides K separate signals(i.e., the N separate signals and the M separate signals) to a signalprocessing portion 570 for further processing. The signal processingportion 570 may include additional spatial and iterative (turbo)processing, as well as de-interleaving (bit and/or symbol level) andchannel decoding.

[0066] Turning now to FIG. 7, a block diagram is provided of amulti-antenna receiver system 700 configured to implement iterativemulti-stage detection in accordance with the present invention. Thereceiver system 700 includes a multistage receiver unit 710 disposed toreceive and process RF signal energy collected by a K element antennaarray 712. As shown, the receiver system also includes a signal recoverymodule 714, which functions to separate K multiplexed channels. In thisway, K symbol streams received at the K element antenna array 712, andconveyed by the K multiplexed channels, may be separated and recoveredat the signal recovery module 714. As shown, the signal recovery portion714 includes an N channel recovery portion 716 and an M channel recoveryportion 718, which cooperate to carry out the functions of the signalrecovery module 714. Specifically, the N channel recovery portion 716 incooperation with the M channel recovery portion 718 function to provideN separate symbol streams and M separate symbol streams, respectively.Together the N separate symbol streams and the M separate symbol streamsprovide K separate symbol streams that correspond to (e.g., closelyestimate) the K symbol streams received at the K element antenna array712.

[0067] As shown, the antenna array 712 includes a first set of Nspatially-separated receiving antennas 704 and a second set of Mspatially-separated receiving antennas 708. The N antennas 704 and the Mantennas 708 couple an RF signal comprised of a first set of N channelsand a second set of M channels into the receiver unit 710. The receivedRF signal is passed through the N antennas 704 to a set #1 channelspreading module 720 and is passed through the M antennas 708 to achannel set #2 spreading module 727. Within the spreading module 720,the N received signal replicas a₁, a₂, . . . a_(N) received from the Nantenna elements 704 ₁, 704 ₂, and 704 _(N) are each spread by adifferent one of N orthogonal sequences of length N associated with thefirst set of N channels.

[0068] Similarly, within the spreading module 727, the M received signalreplicas a_(N+1), a_(N+2), . . . a_(N+M) received from the M antennaelements 70 _(N+1), 708 _(N+2), . . . , 708 _(N+M) are each spread by adifferent one of M orthogonal sequences of length N associated with thesecond set of M channels. A set of N spread signals 730 are provided bythe spreading module 720 to a summation module 731 operative to providea composite set #1 channel signal to a first mixer element 732. In likemanner a set of M spread signals 737 are provided by the spreadingmodule 727 to a summation module 736 operative to provide a compositeset #2 channel signal to a second mixer element 740.

[0069] As shown in FIG. 7, the composite set #1 channel signal isscrambled at the first mixer element 732 using a first PN scramblingsequence P₁ and the composite channel set #2 signal is scrambled at thesecond mixer element 740 using a second PN scrambling sequence P₂. Theresultant set #1 channel and set #2 channel scrambled signals (alsoreferred to herein as a first set of N channel signals and a second setof M channel signals, respectively) are combined within a summationmodule 744 in order to form a multiplexed signal 745 which includes thefirst set of N channel signals and a second set of M channel signals.Within an RF processing module 778 the multiplexed signal 745 isfiltered, down-converted from RF, and digitized to reform themultiplexed signal 745 as a baseband multiplexed signal 746 composedfrom received samples at baseband frequencies.

[0070] The baseband multiplexed signal 746 output of the RF processingmodule 778 is provided to a buffer 749 in the signal recover module 714,and the buffer 749 is switchably coupled to a baseband mixer element 752via a switch 750.

[0071] As shown, the complex conjugate P₁* of the first PN scramblingsequence P₁ is also applied to the baseband mixer element 752 which, incooperation with a set #1 channel despreading module 756, serves todespread the received first set of N channel signals. In particular,within the despreading module 756 the complex conjugates of each of theN orthogonal time sequences are each used to complete the despreading ofthe N baseband signal streams 760 received from the baseband mixerelement 752. That is, each of the N baseband signals is despread by oneof the N orthogonal time sequences. In an exemplary embodiment, thedespreading module 756 includes a bank of N complex correlators whichare matched to the N channels in the first set of N channels. The set ofN despread baseband signals from the despreading module 756 are thenpassed through a corresponding set of N threshold detectors 767, whichyields an initial estimate of the current symbol values for each of thereceived first set of N channel signals (i.e., â₁, â₂, . . . , â_(N)).

[0072] In accordance with the invention, the estimated symbol values â₁,â₁, . . . , â_(N) for the first set of N channel signals are used tosynthesize an interference signal intended to replicate the basebandsignal waveform of the received first set of N channel signals.Specifically, the estimated symbol values â₁, â₂, â_(N) of first set ofN channel signals are processed by a re-spreading module 768 operativeto spread each such value using the applicable one of the N orthogonaltime sequences. The resultant re-spread set of N channel signals arethen combined within a summation module 772 in order to produce acomposite re-spread signal. As shown, the composite re-spread signal isscrambled within mixer element 776 using the first PN sequence P₁,thereby yielding a regenerated set of N channel signals 777, which isprovided as an interference signal 780 to a difference element 782 inthe M channel recovery portion 718. The regenerated set of N channelsignals 777 is also provided to an adder element 798 for use during asecond iteration.

[0073] The difference element 782 is arranged to receive theinterference signal 780 for the first set of N channels and the basebandmultiplexed signal 746 from the delay element 787. The output ofdifference element 782, which approximates the baseband signal waveformof the second set of M channel signals, is descrambled by mixer element786 using the complex conjugate P₂* of the second PN sequence P₂. Theresultant descrambled signal is then despread within the despreadingmodule 788 by each of the M orthogonal time sequences associated withthe second set of M channels. In an exemplary embodiment, thedespreading module 788 includes a bank of M complex correlators whichare matched to the M channels in the first set of M channels. Theresulting set of M despread baseband signals from the despreading module788 are applied to a set of M threshold detectors 790, which yieldestimates of current symbol values â_(N+1), â_(N+2), . . . , â_(N+M) foreach of the second set of M channel signals. The estimated symbol valuesâ₁, â_(N+2), . . . , â_(N+M) of the second set of M channel signals areprocessed by a second re-spreading module 792 operative to spread eachsuch value using the applicable one of the M orthogonal time sequences(i.e., the subset of the N orthogonal sequences used by the channel set#2 spreading module 727). The resultant re-spread set of M channelsignals are then combined within a summation module 794 in order toproduce a second composite re-spread signal. As shown, the secondcomposite re-spread signal is scrambled within mixer element 796 usingthe second PN sequence P₂, thereby yielding a regenerated set of Mchannel signals 797, which is provided to an adder element 798.

[0074] As a consequence, K separate estimated symbol values, i.e., theestimated symbol values â₁, â₂, . ., â_(N) of first set of N channelsignals and the estimated symbol values â_(N+1), â_(N+2), . . . ,â_(N+M) of the second set of M channel signals, are provided during afirst iteration.

[0075] The adder element 798 combines the regenerated set of N channelsignals 777 and the regenerated set of M channel signals 797 to form aregenerated baseband multiplexed signal 799, which according to anexemplary embodiment, is processed during a second iteration asdiscussed herein to produce a more accurate set of K separate symbolvalues.

[0076] The iterative interference removal process will be betterunderstood with a brief consideration of the effect of spreading andscrambling the N and M signal replicas received at the N antennas 505and the M antennas 508, respectively. To begin, suppose {W_(i)|i=1,2, .. . , N} designate the N binary orthogonal time sequences used inspreading the first set of N channel signals. The i^(th) of thesequences may be expressed as W_(i)=(w_(i,1), W_(i,2), . . . , W_(i,N)),where w_(i,m) designates the m^(th) chip of the sequence W_(i). Notethat each of the sequences W_(i) is independent of the symbol index,since each sequence repeats from one symbol to the next. Next, supposethat {P_(n)|i=1,2} designate the first and second PN scramblingsequences that overlay the time orthogonal sequences of the first set ofN channel signals and the second set of M channel signals. Although thefirst and second PN sequences P₁ and P₂ do not repeat, the symbol indexmay also be removed from the PN sequences since the signal processing ofconcern is memoryless. That is, detection of a current symbol does notinvolve signal samples from previous and future symbols. Consequently,each of the PN sequences may be expressed as P_(n)=(p_(n,1), p_(n,2), .. . , p_(n,N)). The resulting composite sequences for channel i (i=1,2,. . . , N) and channel N+k (k=0,2, . . . , M) are denoted (α_(i,1, α)_(i,2), . . . , α_(i,N)) and (β_(i,1), β_(i,2), . . . , β_(i,N))respectively, with α_(i,m)=w_(i,m)p_(1,m) and β_(k,m)=w_(k,m)p_(2,m) form=1,2, . . . , N.

[0077] Since it will be desired to divide the power of the synthesizedinterference signal evenly over the in-phase and quadrature componentsof the useful signal (irrespective of carrier phases), complex-valued PNsequences are considered; that is, the chips p_(n,m) randomly assumevalues from the set {exp(jπ/2), exp(−jπ/2), exp(j3 π/2), exp(−j3π/2)}.

[0078] Referring next to FIG. 8, shown is a flowchart depicting stepscarried out by the multi-antenna receiver system 700 when carrying outthe iterative multistage detection process according to one embodimentof the present invention.

[0079] As mentioned earlier, the interference affecting first set of Nchannels is limited. Accordingly, initial estimates of the symbol valuesof the first set of N channel signals may be made using a thresholddetector immediately following despreading by the correspondingcomposite chip sequences (Step 802). This step of the detection processyields the following set of set of initial decisions for the first setof N channel signals: â₁, â₂, . . . , â_(N).

[0080] The initial decisions â₁, â₂, . . . , â_(N) are then used tosynthesize an estimated interference caused by the first set of Nchannels with respect to the second set of M channels (Step 804). Thisestimated interference is then subtracted from the baseband signalenergy of the multiplexed signal (Step 806), thereby yielding adifference signal corresponding to an estimate of the second set of Mchannel signals (at baseband). Assuming each of the second set of Mchannels may be identified by an index N+k (k=1,2, . . . , M), the totalinterference from the first set of N channels may be expressed as:$\begin{matrix}{I_{N + k} = {\frac{1}{N}{\sum\limits_{i = 1}^{N}\quad {a_{i}\left\lbrack {\sum\limits_{j = 1}^{N}\quad \left( {\alpha_{i,j}\beta_{k,j}^{*}} \right)} \right\rbrack}}}} & (1)\end{matrix}$

[0081] where α_(i) is the data symbol of the i^(th) channel during thecurrent symbol interval. Each term in the outer sum in (1) representsthe interference from one of the N channels. Since the chip sequences(α_(i,1), α_(i,2), . . . , α_(i,N)) and (β_(i,1), β_(i,2), . . . ,β_(i,N)) are known to the receiver, I_(N+k) can be estimated once thesymbol decisions corresponding to channels 1 to N of the first set of Nchannels have been made. This estimate I_(N+k) is subtracted from thecorresponding signal at a correlator output before sending the result toa threshold detector.

[0082] After the estimated interference caused by the first set of Nchannels is removed from the multiplexed signal, the symbol values ofthe received second set of M channel signals are estimated using thethreshold detector 790 immediately following despreading by thedespreading module 788 (Step 808). This step of the detection processyields the following set of symbol decisions for the second set of Mchannel signals: â_(N+1), â_(N+2), . . . , â_(N+M).

[0083] If all initial decisions â₁, â₂, . . . , â_(N) for the first setof N channel signals are made correctly at Step 802, completeinterference cancellation effectively occurs at Step 806 andsubstantially no mutual interference between the first set of N channelsand the second set of M channels will remain when the symbol values ofthe second set of M channel signals are estimated at Step 808. Eachincorrect decision with regard to â₁, â₂, . . . , â_(N) yielded in Step802 will, however, cause the corresponding term in I_(N+k) to increaseand thereby reduce the likelihood of accurate estimation of the secondset of M channels.

[0084] In an exemplary embodiment, to improve the accuracy of detectionof the symbols conveyed by the second set of M channels, a seconditeration of may be performed. Specifically, the symbol decisionsâ_(N+1), â_(N+2), . . . , â_(N+M) made for the second set of M channelsin the first iteration are used to synthesize interference of the secondset of M channel signals (Step 810). The interference of the second setof M channels is then subtracted from the first set of N channel signals(Step 812).

[0085] During the first iteration, the baseband multiplexed signal 746produced by the RF processing module 778 is buffered within buffer 749and directly coupled therefrom to the mixer element 752 via switch 750.During the second and any subsequent iterations, the switch 750 is setto couple the regenerated baseband multiplexed signal 799 from theoutput of adder element 798 (obtained from mixer elements 776 and 796)to the baseband mixer element 752, while the while the buffer 749 isfilled with the incoming signal received from RF processing module 778.Ideally, all iterations are performed while the buffer 749 is updatedand completed before the buffer contents has been filled with a new RFsignal. In other words, the iterative processing is done within one bitinterval (i.e., within one bit duration), so that the size of the buffer749 remains manageable. In an exemplary embodiment, the iterativeprocessing is performed in a much shorter period than a bit duration,and when the buffer 749 is filled with a new set of bit samples, theprocessing of the new set of bit samples by the signal recovery module714 begins.

[0086] The interference from the second set of M channels in the k^(th)channel signal (k=1,2, . . . , N) is given by: $\begin{matrix}{I_{k} = {\frac{1}{N}{\sum\limits_{i = 1}^{N}\quad {a_{N + i}\left\lbrack {\sum\limits_{j = 1}^{N}\left( {\beta_{i,j}\alpha_{k,j}^{*}} \right)} \right\rbrack}}}} & (2)\end{matrix}$

[0087] This interference is synthesized by substituting â_(N+i) forâ_(N+i) in Equation (2) above for i=1,2, . . . , M. Sinceâ_(N+i)=â_(N+i) with a probability close to 1, the synthesized replicawill generally be virtually identical to the actual interference.

[0088] In an exemplary embodiment, during a second iteration, theregenerated baseband multiplexed signal 799 is descrambled and despreadby the baseband mixer element 752 and the set #1 channel despreadingmodule 756 to provide N despread baseband signals. The synthesizedinterference due to the second set of M channel signals (determinedduring the first iteration) from Step 810 is then subtracted from thek^(th) signal of the set of N despread baseband signals at the output ofthe despreading module 756; thus effectively subtracting theinterference of the second set of M channels from the first set of Nchannel signals (Step 812).

[0089] The N interferenced-reduced signals produced by subtracting thesynthesized interference from the k^(th) signal of the set of N despreadbaseband signals is passed to the applicable threshold detector 767.This process is repeated for all of the first set of N channels todetermine a revised set of N symbol values of the first set of N channelsignals (Step 814).

[0090] A revised estimate of the interference caused by the first set ofN channels with respect to the second set of M channels is thendetermined based upon the a revised set of symbol values (Step 816). Inan exemplary embodiment, the revised symbol values of the first set of Nchannel signals are respread by the re-spreading module 768, recombinedwithin the summation module 772 and scrambled within mixer element 776using the first PN sequence P₁, thereby producing another interferencesignal 780, which is subtracted from the regenerated basebandmultiplexed signal at the difference element 782 so as to generate adifference signal corresponding to an estimate of the second set of Mchannel signals (Step 816).

[0091] Symbol value decisions for the second set of M channels are thenmade during the second iteration following subtraction of theinterference of the first set of N channels (Step 818). In this regardthe total interference experienced by the k^(th) channel of the secondset of M channels (i.e., channel N+k) is given by Equation (1). Aftersubtracting the best available estimate of this total interference, theoutput of the despreading module 788 for the k^(th) channel of thesecond set of M channels is sent to the corresponding set of M thresholddetectors 790, which produces a revised set of M symbol values of thefirst set of M channel signals.

[0092] Thus, after the second iteration, a revised set of N symbolvalues of the first set of N channel signals and a revised set of Msymbol values of the second set of M channel signals is provided by thesignal recovery portion 714. Together such revised symbol values provideK separate symbol values, which correspond to K symbol streams in the Kreceived signal replicas received at the K element antenna array 712.

[0093] It has been found when the number of excess channels M is limitedto approximately 25% of the spreading factor N, execution of two orthree iterations yields sufficiently good performance that additionaliterations are unnecessary. As the number of excess channels Mapproaches 25% of N, performance has been found to be improved throughexecution of additional iterations.

Simulation Results

[0094] FIGS. 9-13 depict the results of various simulations of theabove-described iterative multi-stage detection process using two setsof orthogonal spreading sequences. In FIGS. 9-12 a spreading factor N of16 was employed, while in FIG. 13 a spreading factor N of 7 wasutilized. The number of “excess” channels M was selected to be 1, 2, 3and 7 in FIGS. 9-12, respectively, and M was chosen to be 1 in the caseof FIG. 13. In addition, the simulations were executed exclusively atbaseband (no modulation or spectrum-shaping filtering were simulated),and an AWGN channel and synchronous operation (i.e., synchronous timespread) were assumed.

[0095] Referring to FIGS. 9-13, trace A represents the theoreticalsingle user bound while trace B represents the performance of a single,uncoded channel (i.e., as “single user bound”) obtained throughsimulation. In addition, trace C represents the BER of the first set ofN channels prior to the performance of interference cancellation, traceD represents the BER of the second set of M channels prior tointerference cancellation, and trace E illustrates the overall BER(i.e., both the first set of N and the second set of M channels) priorto interference cancellation. The BER of the first set of N channelsfollowing the first iteration of interference cancellation isrepresented by trace F, the BER of the second set of M channelsfollowing the first iteration of interference cancellation isrepresented by trace G, and the overall BER following the firstiteration of interference cancellation is illustrated by trace H.Finally, the BER of the first set of N channels following the seconditeration of interference cancellation is represented by trace I, theBER of the second set of M channels following the second iteration ofinterference cancellation is represented by trace J, and the overall BERfollowing the second iteration of interference cancellation isillustrated by trace K.

[0096] Although the simulations represented by FIGS. 9-13 demonstratethe effectiveness of certain embodiments the inventive iterativemulti-stage detection technique, a value N of 16 (which results indeployment of at least 16 antennas) may be impractical in certainapplications. However, FIG. 13 demonstrates the effectiveness of theinventive technique under currently practical conditions (i.e., N=7,M=1).

[0097] In the simulations of FIGS. 9-13, complex PN scrambling sequenceswere utilized. Specifically, the proposed π/2-separated complexscrambling sequence symbols were replaced with symbols separated by π/7intervals. Other simulations based upon real-valued PN scramblingsequences have not been found to yield performance of similar BER.

[0098] The foregoing description, for purposes of explanation, usedspecific nomenclature to provide a thorough understanding of theinvention. However, it will be apparent to one skilled in the art thatthe specific details are not required in order to practice theinvention. In other instances, well-known circuits and devices are shownin block diagram form in order to avoid unnecessary distraction from theunderlying invention. Thus, the foregoing descriptions of specificembodiments of the present invention are presented for purposes ofillustration and description. They are not intended to be exhaustive orto limit the invention to the precise forms disclosed, obviously manymodifications and variations are possible in view of the aboveteachings. The embodiments were chosen and described in order to bestexplain the principles of the invention and its practical applications,to thereby enable others skilled in the art to best utilize theinvention and various embodiments with various modifications as aresuited to the particular use contemplated.

[0099] For example, an exemplary embodiment was described wherein asymbol level procedure was performed to remove the interference due tothe second set of M channel signals from the first set of N channelsignals at the correlator outputs of despreading module 756. It shouldbe recognized that in an alternative embodiment, interference due to thesecond set of M channel signals from the first set of N channel signalsmay be removed at the chip level using a difference element as was doneto remove the interference due to the first set of N channels from thesecond set of M channel signals using the difference element 782.

[0100] Moreover, in the described exemplary embodiment, interference dueto the first set of N channel signals was removed from the second set ofM channel signals at the chip level using the difference element 782. Inan alternative embodiment, interference due to the first set of Nchannel signals may be removed from the second set of M channel signalson the symbol level at a correlator output after the despreading module788. In other words, interference from either the first set of Nchannels or the second set of M channels may be removed at either thechip or the symbol level and still be well within the scope of thepresent invention.

What is claimed is:
 1. A method for receiving a signal comprising:receiving K replicas of the signal, each of the K replicas beingreceived by one of a corresponding K antennas so as to thereby generateK received signal replicas; processing each of the K received signalreplicas using one of N orthogonal sequences, thereby generating Kprocessed signal replicas, wherein N is less than K; orthogonallymultiplexing the K processed received signal replicas into a multiplexedsignal provided to a signal processing chain; downconverting, within thesignal processing chain, the multiplexed signal into a basebandmultiplexed signal; and transforming the baseband multiplexed signalinto K separate signals wherein each of the K separate signalscorresponds to one of the K replicas of the signal.
 2. The method ofclaim 1 wherein the processing includes: assigning each of N of the Kreceived signal replicas a corresponding one of the N orthogonalsequences so as to thereby generate a first composite signal; scramblingthe first composite signal according to a first scrambling sequence soas to thereby generate a first set of N channel signals; assigning eachof M of the K received signal replicas a corresponding one of Morthogonal sequences so as to thereby generate a second compositesignal, wherein the M orthogonal sequences are a subset of the Northogonal sequences; scrambling the second composite signal accordingto a second scrambling sequence so as to thereby generate a second setof M channel signals; and combining the first set of N channel signalsand the second set of M channel signals so as to generate themultiplexed signal.
 3. The method of claim 2 wherein the transformingincludes: removing interference due to the first set of N channelsignals from the second set of M channel signals, thereby generating Minterference-reduced signals comprising a subset of the K separatesignals.
 4. The method of claim 3 wherein the transforming includes:removing interference due to the second set of M channel signals fromthe first set of N channel signals, thereby generating Ninterference-reduced signals comprising a subset of the K separatesignals.
 5. The method of claim 3 wherein the removing includes:despreading the first set of N channel signals so as to generate a setof N despread baseband signals; synthesizing an interference signal as afunction of the set of N despread baseband signals; and subtracting theinterference signal from the baseband multiplexed signal therebyremoving interference due to the first set of N channel signals from thesecond set of M channel signals.
 6. The method of claim 5 wherein thesynthesizing includes: passing each of the N despread baseband signalsthrough a corresponding one of N threshold detectors so as to generatean estimated set of N symbol values for the first set of N channelsignals; spreading each of the N symbol values according to acorresponding one of the N orthogonal sequences so as to generate afirst baseband composite signal; and scrambling the first basebandcomposite signal according to the first scrambling sequence so as tosynthesize the interference signal.
 7. The method of claim 4 wherein theremoving includes: despreading the first set of N channel signals so asto generate a set of N despread baseband signals; despreading the secondset of M channels signals so as to generate a set of M despread basebandsignals; subtracting, from each of the N despread baseband signals, aninterference signal synthesized as a function of the M despread basebandsignals thereby removing interference due to the second set of M channelsignals from the first set of N channel signals.
 8. The method of claim7 wherein the interference signal is synthesized as a function ofestimated symbol values generated from the M despread baseband signals.9. The method of claim 1 wherein the signal complies with acommunication protocol selected from the group consisting of: orthogonalfrequency division multiplexing (OFDM), time division multiple access(TDMA), code division multiple access (CDMA), gaussian minimum shiftkeying (GMSK), complementary code keying (CCK), quadrature phase shiftkeying (QPSK), frequency shift keying (FSK), phase shift keying (PSK),and quadrature amplitude modulation (QAM).
 10. An apparatus forreceiving a signal comprising: K antenna elements, wherein the K antennaelements are arranged to receive one of a corresponding K replicas ofthe signal and thereby generate K received signal replicas; a signalprocessing chain; a first multiplexer configured to receive N of the Kreceived signal replicas and generate a first set of N channel signals,wherein each of the N channel signals is spread according to acorresponding one of N orthogonal sequences and corresponds to one ofthe N received signal replicas; a second multiplexer configured toreceive M of the K received signal replicas and generate a second set ofM channel signals, wherein each of the M channel signals is spreadaccording to one of the N orthogonal sequences and corresponds to one ofthe M received signal replicas; a summing portion coupled between thesignal processing chain and the first and second multiplexers, whereinthe summing portion is configured to combine the first set of N channelsignals and the second set of M channel signals into a multiplexedsignal and provide the multiplexed signal to the signal processingchain; a downconversion module configured to downconvert, within thesignal processing chain, the multiplexed signal to a basebandmultiplexed signal; and a signal recovery module coupled to the signalprocessing chain, wherein the signal recovery module is configured toreceive the baseband multiplexed signal and provide K separate signalsfrom the baseband multiplexed signal, wherein each of the K separatesignals corresponds to one of the K replicas of the signal.
 11. Theapparatus of claim 10 wherein the first multiplexer includes: a firstspreading module coupled to N of the K antenna elements, wherein thefirst spreading module is configured to receive N of the K receivedsignal replicas and orthogonally spread each of the N received signalreplicas with a corresponding one of the N orthogonal sequences so as togenerate a set of N spread signals a first summing module coupled to thefirst spreading module wherein the first summing module is configured tocombine the set of N spread signals so as to generate a first compositesignal; and a first scrambling portion coupled to the first summingmodule, wherein the first scrambling portion is configured to generatethe first set of N channel signals by scrambling the first compositesignal.
 12. The apparatus of claim 11 wherein the second multiplexerincludes: a second spreading module coupled to M of the K antennaelements, wherein the second spreading module is configured to receive Mof the K received signal replicas and orthogonally spread each of the Mreceived signal replicas with at least one of the N orthogonal sequencesso as to be capable of generating a set of M spread signals; a secondsumming module coupled to the second spreading module wherein the secondsumming module is configured to combine the set of M spread signals soas to be capable of generating a second composite signal; and a secondscrambling portion coupled to the second summing module, wherein thesecond scrambling portion is configured to generate the second set of Mchannel signals by scrambling the second composite signal.
 13. Theapparatus of claim 10 wherein the signal recovery module includes an Nchannel recovery portion configured to provide N separate signals fromthe baseband multiplexed signal, each of the N separate signalscorresponding to one of the N received signal replicas; and an M channelrecovery portion coupled to the N channel recovery portion, wherein theM channel recovery portion is configured to provide M separate signalsfrom the baseband multiplexed signal, each of the M separate signalscorresponding to one of the M received signal replicas; wherein the Kseparate signals include the N separate signals and the M separatesignals.
 14. The apparatus of claim 13 wherein the N channel recoveryportion is configured to provide an interference signal to the M channelrecovery portion, wherein the interference signal is an estimate ofinterference the first set of N channel signals impart upon the secondset of M channel signals; wherein the M channel recovery portion isconfigured to subtract the interference signal from the basebandmultiplexed signal before providing the M separate signals.
 15. Theapparatus of claim 13 wherein the M channel recovery portion isconfigured to provide an interference signal to the N channel recoveryportion, wherein the interference signal is an estimate of interferencethe second set of M channel signals impart upon the first set of Nchannel signals; wherein the N channel recovery portion is configured togenerate N despread baseband signals from the baseband multiplexedsignal and subtract the interference signal from at least one of the Ndespread baseband signals before providing the N separate signals. 16.The apparatus of claim 13 wherein the N channel recovery portionincludes: a first despreading module configured to despread the firstset of N channel signals so as to be capable of generating N despreadbaseband signals; and a set of N threshold detectors, wherein each ofthe N threshold detectors is coupled to the first despreading module soas to receive a corresponding one of the N despread baseband signals,wherein each of the N threshold detectors provides a symbol estimate fora corresponding one of the N separate signals so as to generate N symbolestimates.
 17. The apparatus of claim 16 wherein the N channel recoveryportion includes: a first respreading portion coupled to the set of Nthreshold detectors, wherein the first respreading portion is configuredto receive the N symbol estimates and spread each of the N symbolestimates according to a corresponding one of the N orthogonal sequencesso as to generate a first set of N spread symbol estimates; a firstbaseband summing portion coupled to the first respreading portion,wherein the first baseband summing portion is configured to combine theN spread symbol estimates so as to be capable of generating a firstbaseband composite signal; and a first baseband scrambling portioncoupled to the first baseband summing portion, wherein the firstbaseband scrambling portion is configured to receive the first basebandcomposite signal and scramble the first baseband composite signal so asto be capable of generating an interference signal; wherein theinterference signal is an estimate of the interference from the firstset of N channel signals imparted upon the second set of M channelsignals.
 18. The apparatus of claim 17 wherein the M channel recoveryportion includes: a difference element coupled between thedownconversion module and the first baseband scrambling portion, whereinthe difference element is configured receive the interference signalfrom the first baseband scrambling portion and the baseband multiplexedsignal from the downconversion module, wherein the difference element isconfigured to subtract the interference signal from the basebandmultiplexed signal so as to be capable of removing interference that thefirst set of N channel signals imparts upon the second set of M channelsignals.
 19. The apparatus of claim 10 wherein the signal complies witha communication protocol selected from the group consisting of:orthogonal frequency division multiplexing (OFDM), time divisionmultiple access (TDMA), code division multiple access (CDMA), gaussianminimum shift keying (GMSK), complementary code keying (CCK), quadraturephase shift keying (QPSK), frequency shift keying (FSK), phase shiftkeying (PSK), and quadrature amplitude modulation (QAM).
 20. Anapparatus for receiving a signal comprising: an antenna array comprisingK antenna elements, wherein the K antenna elements are spatiallyarranged to receive one of a corresponding K replicas of the signal, soas to be capable of generating K received signal replicas; a signalprocessing chain; means for processing each of the K received signalreplicas using one of N orthogonal sequences, so as to thereby generateK processed signal replicas, wherein N is less than K means fororthogonally multiplexing the K processed received signal replicas intoa multiplexed signal provided to the signal processing chain; means fordownconverting, within the signal processing chain, the multiplexedsignal into a baseband multiplexed signal; and means for transformingthe baseband multiplexed signal into K separate signals wherein each ofthe K separate signals corresponds to one of the K replicas of thesignal.
 21. The apparatus of claim 20 wherein the means for processingincludes: means for assigning each of N of the K received signalreplicas a corresponding one of the N orthogonal sequences so as to becapable of generating a first composite signal; means for scrambling thefirst composite signal according to a first scrambling sequence so as tocapable of generating a first set of N channel signals; means forassigning each of M of the K received signal replicas a correspondingone of M orthogonal sequences so as to be capable of generating a secondcomposite signal, wherein the M orthogonal sequences are a subset of theN orthogonal sequences; means for scrambling the second composite signalaccording to a second scrambling sequence so as to be capable ofgenerating a second set of M channel signals; and means for combiningthe first set of N channel signals and the second set of M channelsignals so as to be capable of generating the multiplexed signal. 22.The apparatus of claim 21 wherein the means for transforming includes:means for removing interference due to the first set of N channelsignals from the second set of M channel signals so as to therebygenerate M interference-reduced signals comprising a subset of the Kseparate signals.
 23. The apparatus of claim 22 wherein the means fortransforming includes: means for removing interference due to the secondset of M channel signals from the first set of N channel signals so asto thereby generate N interference-reduced signals comprising a subsetof the K separate signals
 24. The apparatus of claim 22 wherein themeans for removing includes: means for despreading the first set of Nchannel signals so as to be capable of generating a set of N despreadbaseband signals; means for synthesizing an interference signal as afunction of the set of N despread baseband signals; and means forsubtracting the interference signal from the baseband multiplexed signalso as to be capable of removing interference due to the first set of Nchannel signals from the second set of M channel signals.
 25. Theapparatus of claim 24 wherein the means for synthesizing includes: meansfor generating an estimated set of N symbol values for the first set ofN channel signals as a function of the N despread baseband signals;means for spreading each of the N symbol values according to acorresponding one of the N orthogonal sequences so as to be capable ofgenerating a first baseband composite signal; and means for scramblingthe first baseband composite signal according to the first scramblingsequence so as to be capable of synthesizing the interference signal.26. The apparatus of claim 23 wherein the means for removing includes:means for despreading the first set of N channel signals so as to becapable of generating a set of N despread baseband signals; means fordespreading the second set of M channels signals so as to be capable ofgenerating a set of M despread baseband signals; means for synthesizingan interference signal as a function of the M despread baseband signals;and means for subtracting, from each of the N despread baseband signals,the interference signal so as to be capable of removing interference dueto the second set of M channel signals from the first set of N channelsignals.
 27. The apparatus of claim 26 wherein the means forsynthesizing the interference signal includes means for generatingestimated symbol values from the M despread baseband signals wherein themeans for synthesizing the interference signal includes means forsynthesizing the interference signal as a function of the estimatedsymbol values.
 28. The apparatus of claim 20 wherein the signal complieswith a communication protocol selected from the group consisting of:orthogonal frequency division multiplexing (OFDM), time divisionmultiple access (TDMA), code division multiple access (CDMA), gaussianminimum shift keying (GMSK), complementary code keying (CCK), quadraturephase shift keying (QPSK), frequency shift keying (FSK), phase shiftkeying (PSK), and quadrature amplitude modulation (QAM).
 29. A methodfor multiplexing K channels on to a receiver chain, the K channelsincluding N channels corresponding to N antenna elements and M channelscorresponding to M antenna elements, the method comprising: spreadingeach of the N channels according to a corresponding one of N orthogonalsequences so as to form N spread channels; overlaying a first scramblingsequence on to the N spread channels so as to form a first set of Nchannels; spreading each of the M channels according to one of the Northogonal sequences so as to form M spread channels; overlaying asecond scrambling sequence on to the M spread channels so as to form asecond set of M channels; combining the first set of N channels and thesecond set of M channels so as to form K multiplexed channels; andproviding the K multiplexed channels to the receiver chain.
 30. A methodfor separating K symbol streams, each of the K symbol streams beingconveyed by K respective orthogonally spread channels in a receiverchain, the K channels including a first set of N channels and a secondset of M channels, each of the N channels being spread according to acorresponding one of N orthogonal sequences and each of the M channelsbeing spread according to one of the N orthogonal sequences, the methodcomprising: despreading the first set of N channels so as to generate Nseparate channels; detecting, from the N separate channels, a set of Nsymbols wherein each of the N symbols is conveyed by a corresponding oneof the N channels; generating a first interference signal due to thefirst set of N channels based upon the set of N symbols; subtracting theinterference signal from the second set of M channels; despreading thesecond set of M channels so as to generate M separate channels;detecting, from the M separate channels, a set of M symbols wherein eachof the M symbols is conveyed by a corresponding one of the M channels;and providing K separate symbols wherein the K separate symbols includethe set of N symbols and the set of M symbols.
 31. A method forreceiving a signal with an antenna array comprising: receiving Kreplicas of the signal, each of the K replicas being received by one ofa corresponding K antenna elements of the antenna array, wherein the Kreplicas include N replicas and M other replicas of the received signal;multiplexing the N replicas and the M replicas of the signal into amultiplexed signal provided to a single processing chain; removinginterference due to the N signals from the multiplexed signal;demultiplexing, after the interference due to the N signals is removed,the M signals from the multiplexed signal, thereby generating M detectedsignals; removing interference due to the M signals from the multiplexedsignal; demultiplexing, after the interference due to the M signals isremoved, the N signals from the multiplexed signal, thereby generating Ndetected signals.